A fuel cell is defined as an electricity-generating cell that generates electricity through combination of hydrogen and oxygen. Unlike general cells such as dry cells, storage cells and the like, fuel cells have advantages in that they can keep generating electricity for as long as hydrogen and oxygen are supplied, and are free from heat loss and thus have about twice the efficiency of internal combustion engines. In addition, since fuel cells directly convert chemical energy, generated by combination of hydrogen and oxygen, to electrical energy, they release almost no contaminants. Accordingly, fuel cells have other advantages of environmental friendliness and the ability to reduce concerns about depletion of resources in accordance with an increase in energy consumption.
Based on the type of electrolyte used, fuel cells are largely classified into polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), alkaline fuel cells (AFCs) and the like. These various types of fuel cells essentially operate on the same principle, but are different in terms of the type of fuel used, operation temperature, catalyst, electrolyte and the like. Of these, polymer electrolyte fuel cells are known to be the most promising in the fields of transport systems as well as small-scale stationary electricity-generators, because they operate at lower temperatures and are capable of realizing miniaturization due to their higher power density, as compared to other fuel cells.
One of the most important factors in improving the performance of polymer electrolyte fuel cells is to maintain moisture content in the polymer electrolyte membrane of membrane electrode assembly by providing moisture in an amount not less than a predetermined level. This is the reason that polymer electrolyte membranes show a rapid decrease in electricity-generation efficiency, when dried.
Methods for humidifying polymer electrolyte membranes include 1) a bubbler humidification method that supplies moisture by filling a internal pressure vessel with water and passing a target gas through a diffuser, 2) a direct injection method that supplies moisture by calculating a moisture supply amount required for the reaction of fuel cells and directly supplying the amount of moisture through a solenoid valve to a gas flow pipe, 3) a humidifying-membrane method that supplies moisture to a gas flow layer using a polymeric separation membrane, and the like. Of these, the humidifying-membrane method, which humidifies a polymer electrolyte membrane by supplying water vapor to a gas provided to the polymeric electrolyte membrane using a membrane that selectively permeates water vapor contained in an exhaust gas, is highly advantageous in that a humidifier can be light-weight and miniaturized.
The selective permeation membrane used for the humidification membrane method is preferably a hollow fiber membrane which has a high index of permeation area per unit volume in case of module formation. That is, the use of a hollow fiber membrane for production of humidifiers has advantages in that fuel cells can be sufficiently humidified even with a small amount due to the possibility of high-integration of the hollow fiber membrane with a wide contact surface area, inexpensive materials are available, and moisture and heat contained in an unreacted hot gas discharged from fuel cells can be recovered and then recycled through the humidifier.
Meanwhile, fuel cells, in particular, fuel cells for transport systems, must have sufficient durability to operate for extended periods of time. Accordingly, hollow fiber membranes for humidifiers require moisture and heat resistance to avoid deterioration for a long time under conditions of high temperature and high humidity. In addition, hollow fiber membranes for humidifiers should have superior hydrophilicity to selectively permeate moisture contained in an exhaust gas. However, moisture and heat resistance and hydrophilicity are mutually exclusive properties which are incompatible with each other. Accordingly, hollow fiber membranes for humidifiers that satisfy the moisture and heat resistance have a disadvantage of unsatisfactory humidifying performance due to low hydrophilicity. In contrast, hollow fiber membranes for humidifiers that exhibit superior humidifying performance due to high hydrophilicity have low moisture and heat resistance, thus being disadvantageously unsuitable for the case where humidifiers should operate for a long period of time.